As part of their quality assurance, semiconductor device makers systematically perform tests on their products to ensure that they meet or exceed all of their design parameters. Among the types of tests routinely performed include device parametric testing (a.k.a. DC testing), device logic function testing, and device timing testing (a.k.a. AC testing). While the semiconductor device being tested is often referred to as the Device Under Test, the test system used in conducting the above tests on the DUT is often referred to as Automatic Test Equipment (ATE).
The ATE is necessarily very precise to carry out the aforementioned tests on very sensitive DUT like semiconductor devices. In general, the ATE hardware is controlled by a computer which executes a test program to present the correct voltages, currents, timings, and functional states to the DUT and monitor the response from the device for each test. The result of each test is then compared to pre-defined limits and a pass/fail decision is made. As such, the ATE hardware normally include a collection of power-supplies, meters, signal generators, pattern generators, etc.
The Pin Electronics (PE) circuitry provides the interface between the ATE and the DUT. More particularly, the PE circuitry supplies input signals to the DUT and receives output signals from the DUT. As an example, in parametric testing, either an input voltage is sent to the DUT and an output current is received from the DUT or an input current is sent to the DUT and an output voltage is received from the DUT. Accordingly, a programmable current source is one of the PE's required components to drive desired currents to the DUT.
FIG. 1 illustrates, as an example, a prior art current source used in a PE circuitry. As shown in FIG. 1, prior art current source 100 comprises digital-to-analog (D/A) converter 101, bipolar transistors 102-103, and resistor R.sub.Iset. D/A converter 101 receives as inputs an analog reference voltage V.sub.refin and a digital programmed value PV from the test computer. In response, D/A converter 101 outputs an analog voltage V.sub.out. The output of D/A converter 101 is connected to resistor R.sub.iset which in turn is connected to the collector of transistor 102. The base of transistor 102 is connected to the base of transistor 103. Moreover, the base of transistor 102 is also connected to the collector of transistor 102. The emitter of transistor 102 is connected to a power voltage V.sub.ref. While the emitter of transistor 103 is also connected to voltage V.sub.ref, the collector of transistor 103 supplies the output current I.sub.out of current source 100.
In so doing, transistors 102-103 and resistor R.sub.iset form a current mirror wherein a current is drawn away from the collector of transistor 102 which causes an emitter-collector current to flow. Because transistors 102 and 103 are identical, a substantially equal emitter-collector current is provided as I.sub.out. Examining transistor 102, from Kirchoff's voltage law: EQU V.sub.EB +V.sub.BC +V.sub.CE =0
Because the base is connected to the collector, V.sub.BC =0. As such, the above equation becomes EQU V.sub.CE =-V.sub.EB (1)
From Ohm's law, EQU I.sub.1 =(V.sub.ref -V.sub.BE -V.sub.out)/R.sub.iset (2)
Well-known programmable D/A converter functional characteristics dictate that EQU V.sub.out =V.sub.ref *(PV/FS) (3)
where PV is the digital programmed value and FS is the full scale digital value of the D/A converter.
Substituting equation (3) into equation (2), EQU I.sub.1 =((V.sub.ref -V.sub.BE)-(V.sub.ref *(PV/FS)))/R.sub.iset =(V.sub.ref *(1-(PV/FS))-V.sub.BE)/R.sub.iset (4)
From Kirchoff's current law, EQU I.sub.Emitter +I.sub.Base +I.sub.Collector =0
Current I.sub.Base is approximately equal to I.sub.Emitter /H.sub.fe, where H.sub.fe is the transistor gain which is typically in the range of 150-300. Therefore, I.sub.Base is negligible compared to I.sub.Emitter and I.sub.Collector. For this reason, EQU -I.sub.Emitter =I.sub.Collector =I.sub.1 (5)
Equation (5) is applicable to both transistors 102 and 103. Because I.sub.E for both transistors 102 and 103 are the same, EQU I.sub.1 =I.sub.out (6)
Since it is well known that V.sub.BE is related to temperature according to the equation: EQU I.sub.E .about.exp(qV.sub.BE /kT) (7)
wherein q is the electronic charge, k is Boltzmann's constant, and T is temperature. Solving equation (7) for V.sub.BE, EQU V.sub.BE .apprxeq.(kT/q)ln(I.sub.E) (8)
As can be seen from equation (4), I.sub.out depends on V.sub.BE. Thus, under prior art current source 100, the output current I.sub.out is affected by temperature variations which in turn affect the precision of the current source. Moreover, prior art current source 100 error .DELTA.V.sub.BE @.DELTA.T is constant over the full operating range, as shown in FIG. 1A, making it impossible to accurately program small values. This can be illustrated by the following example. Assume that V.sub.Ref =5V, I.sub.0 =1 mA, V.sub.BE =0.6V, and that the D/A converter is a 12-bit converter. The resolution for this 12-bit D/A converter is 5V/2.sup.12 bit=1.22 mV/bit. From equation (8), the change .DELTA.V.sub.BE with respect to temperature variations can be determined. However, for silicon as a material, it is common knowledge that .DELTA.V.sub.BE =-2.5 mV/.degree. C. Thus, a change of 1.degree. C. represents a 200% error at the minimum current setting. Following this logic, a change of 25.degree. C.=-62.5 mV which translates to an error equal in magnitude to the lower 6 bits of a 12-bit D/A converter.
Referring now to FIG. 2 illustrating another prior art current source. As shown in FIG. 2, prior art current source 200 consists of a differential amplifier whose output is connected to the bases of the transistors in a current mirror circuit. The differential amplifier consists of operational amplifier (op-amp) 201, resistor R.sub.I 202, resistor R.sub.F 203, resistor R.sub.I 204, and resistor R.sub.F 205. Resistors R.sub.I 202 and R.sub.F 203 are connected in parallel to the non-inverted input of op-amp 201. Resistor R.sub.I 202 is in turn connected to reference voltage V.sub.Ref. Conversely, resistor R.sub.F 203 is in turn connected to ground. Resistor R.sub.I 204 and R.sub.F 205 are connected in parallel to the inverted input of op-amp 201. Resistor R.sub.I 204 is in turn connected to a voltage source V.sub.I. Resistor R.sub.F 205 is in turn connected to the output of op-amp 201.
The output of op-amp 201 is connected to resistor R.sub.set 206 which in turn is connected to the collector of transistor 207 of the current mirror. The bases of transistors 207 and 208 are connected together as well as to the collector of transistor 207. The emitters of transistors 207 and 208 are connected together as well as to voltage V.sub.+. Finally, the collector of transistor 208 provides the output current for current source 200.
An circuit analysis of current source 200 shows that: EQU I.sub.1 .apprxeq.I.sub.2 .apprxeq.I.sub.out I.sub.out =(V.sub.+ -V.sub.BE1 -V.sub.out)/R.sub.set (9)
where V.sub.BE1 is the base-emitter voltage of transistor 207 and V.sub.out is the output voltage of op-amp 201.
Since voltage V.sub.out is also the output voltage of the differential amplifier, EQU V.sub.out =(V.sub.Ref -V.sub.i)*(R.sub.F /R.sub.I) (10)
Substituting equation (10) into equation (9), the output current is defined as: EQU I.sub.out =(V.sub.+ -V.sub.BE -(V.sub.Ref -V.sub.i))*(R.sub.F /R.sub.I)/R.sub.set (11)
where V.sub.BE =V.sub.BE1 =V.sub.BE2.
Accordingly, like prior art current source 100, prior art current source 200 depends on voltage V.sub.BE which is subject to changes due to temperature variations which in turn greatly affect the precision of the current source. As demonstrated earlier, a change of 1.degree. C. represents a 200% error at the minimum current setting. Moreover, prior art current source 200 error is constant over the full operating range making it impossible to accurately program small values.
On the other hand, U.S. Pat. No. 4,251,743 issued Feb. 17, 1981 to Hareyama (hereinafter Hareyama) discloses a current source designed for used in an Analog-to-Digital (A/D) converter which compensates for temperature variations as well as changes of components' characteristics such as aging. The current source disclosed in Hareyama also implements the current mirror concept. However, the current source disclosed in Hareyama implements feedback control (i.e., closed loop control) of its output current I.sub.out to compensate for errors. As a result, in addition to requiring more hardware, the current source disclosed in Hareyama requires may not be as precise and responsive as desired due to the inherent characteristics (e.g., residual error and time lag) of feedback control.
Thus, a need exists for a precise current source circuit for use in a computer controlled ATE which has good dynamic range that is able to cancel out or compensate for current changes caused by temperature variations.